This invention relates to an apparatus for casting molten metals such as aluminum or magnesium. More specifically, it relates to an improved shot sleeve assembly for use in casting these molten metals. Yet more specifically, it relates to an improvement of the shot sleeve assembly disclosed in my U.S. Pat. No. 4,926,926.
A shot sleeve is a device for injecting molten metals such as magnesium and aluminum into a die or mold. Relatively simple in construction, it typically comprises a steel sleeve defining an axial bore and a piston slidably fitted in the bore to act as an injection ram. An aperture in the wall of the sleeve opens into a portion of the cylinder bore just in front of the piston when it is in the rest position. This aperture is called a xe2x80x9cwellxe2x80x9d and the molten metal is poured into the well for temporary residence in the bore before the piston is actuated.
Because of the high temperature difference between the molten casting metal and the steel sleeve, it is important to minimize the temperature build up in the sleeve during the casting process. It should be understood that the steel sleeve is not heating uniformly because the molten metal lays primarily in the bottom half of the sleeve.
This unequal heating of the sleeve bore and subsequent distortion causes piston wear. The flow path of the metal exiting the sleeve mouth also heats the sleeve bore causing distortion which, again, affects piston life and the effectiveness of the compression on the molten or semisolid metal in the biscuit of metal remaining in the sleeve bore at the end of the piston injection stroke.
Many arrangements have been provided in the past to provide controlled cooling. For example, controlled cooling is achieved in the shot sleeve assembly of my U.S. Pat. No. 4,926,926 by the provision of a copper cladding layer over the die end of the shot sleeve. According to a further cooling technique, a spiral groove is provided around the die end of the sleeve and coolant such as water is circulated through the groove in a controlled manner. Although this arrangement is generally effective to provide the desired cooling, it inherently provides the same magnitude of cooling with respect to both the top and bottom regions of the sleeve bore. However, between pouring of the molten metal and actuation of the piston, the molten metal may sit in the bottom of the horizontal sleeve for several seconds, for example 2-10 seconds depending on the shot weight and other parameters. The molten metal residing in the bottom of the sleeve of course undergoes cooling during this time. If the sleeve is thereafter cooled too aggressively, the portion of the molten metal residing in the lower region of the sleeve may be cooled to a point of freezing prior to injection into the die. This is of course counterproductive to the casting process and the quality of the castings being made. Accordingly, the spiral groove is positioned some distance from the working inside diameter of the sleeve bore to prevent metal chill.
This invention is directed to the provision of a shot sleeve assembly having improved provision for cooling.
More specifically, this invention is directed to the provision of a cooling arrangement for a shot sleeve assembly wherein the cooling around the circumference of the bore is selectively controlled to avoid solidification while retaining the overall benefits of cooling.
The shot sleeve assembly of the invention is designed for moving molten metal into a mold cavity and includes an elongated horizontal shot sleeve having a bore extending axially therethrough from a first end to a second end, adapted to be positioned proximate the mold cavity, and a well opening extending through an upper region of the sleeve at a location adjacent the first end; an injection piston slidably mounted in the bore; and a cooling cell positioned over the second end of the sleeve to cool the molten metal moving through the bore.
According to the invention, the cooling cell defines a continuous coolant passage extending from a coolant inlet to a coolant outlet and the passage presents more surface area in surrounding relation to an upper circumferential region of the bore than to a lower circumferential region of the bore. This arrangement provides selective cooling of the bore whereby to provide less cooling to the lower circumferential region of the bore so as to avoid solidification of metal residing in the lower region of the bore.
According to a further of the invention, the inlet and outlet are in a lower region of the cell and the passage includes an inlet passage region proximate the inlet, an outlet passage region proximate the outlet, and a central passage region interconnecting the inlet and outlet passage regions and extending upwardly and around the upper circumferential region of the bore; and the central passage region has a larger axial width than the inlet and outlet passage regions whereby to present more cooling surface area proximate the upper circumferential region of the bore than proximate the lower circumferential region of the bore. This specific configuration provides a ready and efficient means of selectively varying the extent of cooling provided to the upper and lower regions of the bore. In the disclosed embodiment of the invention, the bore and cooling cell have a circular cross-sectional configuration and the passage is arcuate and is centered on the axis of the bore.
According to a further feature of the invention, the shot sleeve assembly further includes a copper cladding layer, of the type, for example, shown in my U.S. Pat. No. 4,926,926, interposed between the shot sleeve and the cooling cell. The copper cladding layer augments the cooling capacity of the shot sleeve and, specifically, acts to quickly even the distribution of heat around the sleeve diameter and transfer heat to the cooling cell. By creating an even distribution of heat, the bore will stay round and true.
According to a further feature of the invention, the copper cladding layer further acts to facilitate the compression fitting of the cooling cell on the sleeve and specifically, by virtue of its higher coefficient of thermal expansion as compared to the ferrous metal of the shot sleeve, expands when heated to ensure a compression fit of the cooling cell on the sleeve.
According to a further feature of the invention, the cooling cell is formed of an inner cell sleeve fitted over the shot sleeve and an outer cell sleeve fitted over the inner cell sleeve and coacting at its inner periphery with the outer periphery of the inner cell sleeve to define the coolant passage. This arrangement facilitates the provision of a coolant passage having the desired circumferentially selective configuration.
In the disclosed embodiment of the invention, the inlet and outlet extend through a bottom region of a side wall of the outer cell sleeve for communication with the coolant passage. This specific inlet and outlet location allows the coolant passage to extend in a continuous circular fashion around substantially the entire circumference of the shot sleeve. However, the inlet and outlet may alternatively be located on either side of the sleeve diameter; at the pour hole end of the sleeve; or at the top of the sleeve.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.